How sure are you that the green egg is hers? Is there any chance the egg was already in the nest and hidden down in the bedding. Did her squirming and scratching bring it into the open? I've had eggs hidden in the bedding and I've had hens go onto a nest and not lay an egg. I can guess what you are going to say but I have to ask.
Where did you get that Silkie? What do you know of her genetic background? For a hen to lay a green egg at least one of her parents had to hatch from a blue or green egg. That's just the way the genetics works. Silkies are often mixes, not necessarily a pure breed. Somewhere in her background that blue egg gene was introduced if she is laying green eggs.